1. Field of the Invention
The present invention relates to pressure indicating devices, and, more particularly, to a tire pressure indicator which is arranged to remain mounted on the tire by serving as a tire valve cap.
2. Description of the Prior Art
The maintenance of the prescribed tire pressure of an automobile is important, both from a standpoint of traffic safety and from a standpoint of tread wear minimization. It has therefore already been suggested that each tire be provided with a pressure indicating device which can remain attached to the tire valve and which shows, upon simple inspection by the driver, whether or not the tire pressure is still adequate.
The devices which have been suggested for this purpose in the past rely, without exception, on visual inspection as a means to ascertain the pressure condition. However, such visual inspection becomes difficult or impossible, if road dirt has collected on the device, or if the particular wheel happens to be in a position in which the valve is oriented upside down. Most prior art devices have the additional disadvantage that they are comparatively large and heavy, thereby distorting the wheel balance and increasing the risk of damage to the device by a high curb.
A small, lightweight prior art valve cap with a pressure indicator is disclosed in German Gebrauchsmuster (Utility Model) No. 75 34 979. It has a diaphragm-type spring-loaded piston arranged inside a closed housing, the piston being connected to an indicator pin which protrudes axially from one end of the housing. The axial position of the indicator pin is indicative of the tire pressure. This indicator device is designed to provide a tactile as well as a visual reading, having a threaded sleeve which, at the proper tire pressure, can be adjusted to be flush with the indicator pin. A pressure deficit and a corresponding retraction of the pin can be determined by feeling the indicator with a finger, the inability of touching a small prong on the indicator pin meaning that the tire pressure has dropped below the recommended pressure range. This prior art device has the disadvantage of a comparatively short displacement stroke of the indicator pin, meaning that its reading is not very sensitive, so that the tire pressure may drop below the recommended pressure range, without being readily noticed.